A compound semiconductor single crystal is generally prepared by a horizontal Bridgeman method (HB method) or a liquid encapsulated Czochralski method (LEC method).
When a single crystal is prepared according to the LEC method, such a single crystal is pulled by a rotary pulling shaft from a raw material molten solution having a surface covered with a liquid sealant. An apparatus for such pulling comprises an airtight housing, a rotatable, vertically movable upper shaft having a lower end on which a seed crystal is mounted, a crucible for containing a raw material, a lower shaft for supporting the crucible, an evacuator, an inert gas introducing system, a heater, and the like. In such apparatus, the upper and lower shafts are rotated for providing uniform temperature conditions in the direction of rotation. In addition, the raw material molten solution is covered with a liquid sealant and supplied with a high pressure inert gas, in order to suppress decomposition and evaporation of a raw material element having a high vapor pressure near its melting point.
When a single crystal of a compound semiconductor or the like is prepared by a pulling method, generally employed is a method of filling up a crucible with a raw material polycrystal or a raw material single element or impurity only once at the start and supplying no raw material in an intermediate stage. In such a method, the size of the pulled single crystal is inevitably restricted by the amount of the raw material which is filled into the crucible at the start. In an apparatus for carrying out such a method, the raw material molten solution contained in the crucible is reduced as the pulling proceeds. When the quantity of the molten solution is less than a prescribed value, it is no longer possible to pull a single crystal. If a long single crystal can be grown through single pulling, it is possible to reduce loss at each end of the crystal, while it is also possible to reduce time loss in a preparation step for the crystal growth, an extraction step after the growth and the like. Thus, the cost can be remarkably reduced as compared with a case of growing short crystals repeatedly.
In order to pull a longer single crystal through a single growth step, the crucible must be continuously supplied with the raw material. As to a method of pulling a silicon single crystal, methods enabling a continuous supply of raw materials have been proposed in British Patent No. 755,422 (Aug. 22, 1956), Japanese Patent Laying-Open No. 59-79000 (May 8, 1984), U.S. Pat. No. 4,659,421 (Apr. 21, 1987), U.S. Pat. No. 2,977,258 (Mar. 28, 1961) and U.S. Pat. No. 4,650,540 (Mar. 17, 1987), for example.
Each of these methods utilizes a wide crucible to pull a single crystal from a certain pulling region of the crucible while dissolving a solid raw material in another region of the crucible for supplying the same into the pulling region. When the supply quantity of the solid raw material is substantially equal to the pulling quantity, the crucible is regularly provided therein with a constant quantity of the raw material molten solution. Thus, it is possible to pull a silicon single crystal until the solid raw material is used up or the pulling reaches the geometrical limit of the upper shaft in the apparatus. Since it is possible to grow a sufficiently long ingot of a silicon single crystal, the cost for preparing the single crystal can be reduced.
Each of these apparatuses is adapted to pull a single crystal from and supply a solid raw material into the same crucible. The crucible is formed to have a wide surface area and a thin bottom. While the crucible is rotatable about its central axis, the center of the pulling shaft is not aligned with that of the lower shaft supporting the crucible. However, the temperature of the raw material molten solution is uniformalized by stirring since the crucible is being rotated. In the case of a silicon single crystal, it is possible to relatively easily grow a long single crystal by supplying a raw material simultaneously with pulling, as described above. In practice, a silicon single crystal for a wafer of 152 mmU or 203 mmU having a length of at least 1m has been pulled by such a method.
In the case of a group III and V compound semiconductor single crystal, however, it is difficult to apply the aforementioned method In general, the surface of a raw material molten solution is covered with a liquid sealant in order to suppress dissociation of a group V element having a high vapor pressure In order to supply a solid raw material as described above, it is necessary to prevent evaporation of the group V element from this solid. Therefore, the solid raw material to be supplied must be covered with a liquid sealant, for example.
*1* Japanese Patent Laying-Open No. 61-158897 (application filed on Dec. 29, 1984) and *2* Japanese Patent Laying-Open No. 60-137891 (application filed on Dec. 24, 1983) by the applicants discloses methods or apparatuses for preparing single crystals of groups III and V compound semiconductors. FIG. 4 schematically illustrates a concrete example of apparatuses proposed in the above specifications A rotatable crucible 60 contains a GaAs molten solution 61 and B.sub.2 O.sub.3 62 serving as a liquid sealant. This crucible 60 is rotatably supported by a lower shaft. A GaAs single crystal 63 is pulled from the GaAs molten solution 61. A cylinder 64 is provided in a peripheral edge portion of the crucible 60. This cylinder 64 is dipped into the molten solution 61, while a polycrystalline rod 66 of GaAs is inserted therein. The polycrystalline rod 66 is entirely covered with a liquid sealant 65 which is filled into the cylinder 64. Further, an auxiliary heater 68 is provided around the cylinder 64, in order to prevent solidification of the liquid sealant 65. In this apparatus, supply of the raw material molten solution is carried out by gradually moving down the polycrystalline rod 66 and dissolving the same into the molten solution 61. The aforementioned specification *1* shows that a long non-doped GaAs single crystal is pulled while a continuous supply of a raw material takes place. The specification *2* shows that an increase in the impurity concentration was prevented by the supply of a non-doped polycrystal in order to uniformly dope an impurity having a segregation coefficient of not more than 1.
Further, *3* Japanese Patent Laying-Open No. 61-158896 (application filed on Dec. 29, 1984) discloses a method and an apparatus in which Ga and As are added to a raw material molten solution from different paths in place of a solid raw material. FIG. 5 is a typical diagram showing a concrete example of an apparatus described in this specification. A container 71 containing As is mounted under a crucible 70. A small hole 72 is formed in the bottom portion of the crucible 70, in order to introduce As vapor from the container 71. As is supplied into a raw material molten solution 73 through the small hole 72. A Ga container 74 is provided above the crucible 70. In the crucible 70, the supplied As and Ga react with each other to provide a GaAs molten solution. A single crystal is directly pulled from the raw material molten solution synthesized in such a manner.
In each of the methods and apparatuses shown in the above specifications *1* and *2*, it is necessary to perform a prior synthesizing of a polycrystal serving as a raw material to be supplied into the melt. Since the raw material to be supplied is solid, the following disadvantages take place:
(1) The cost for synthesizing a group III and V compound semiconductor raw material is increased.
(2) During the step of synthesizing the supplied polycrystalline raw material may entrap impurities Thus, the raw material molten solution may be contaminated with such impurities contained in the raw material being supplied.
(3) In the apparatus shown in FIG. 4, the raw material solid to be supplied must be sealed in the cylinder with the liquid sealant. Due to geometrical limitations of such a sealing mechanism, the solid to be supplied may not be much increased in size. Since the supply quantity of the raw material is thus limited, the length of the pulled single crystal is also limited.
(4) In the pulling apparatus, a supply mechanism, an auxiliary heater and the like must be arranged in a narrow space above the crucible, and hence the apparatus is complicated in structure.
In the method shown in specification *3*, no expense is required for a prior synthesizing of a polycrystalline raw material since the supply raw material is directly synthesized in the apparatus. However, the method has the following problems:
(1) Heat generated in the synthesis of the raw material molten solution in the crucible disturbs the temperature environment in the molten solution. This exerts a bad influence on the state of the solid-liquid interface.
(2) It is difficult to form a small hole which allows passage of As vapor with no leakage of the raw material molten solution from the bottom portion of the crucible.
(3) While the synthesis quantity of the raw material molten solution is controlled by the supply quantity of Ga, it is difficult to stoichiometrically control As.